A major factor limiting photosynthetic efficiency is the competitive inhibition of CO2 fixation by oxygen, due to lack of specificity of the enzyme RuBisCO. Incorporation of oxygen by RuBisCO is the first-dedicated step in photorespiration, a pathway that respires CO2, compounding photosynthetic inefficiency. Overall, photorespiration reduces photosynthetic productivity by as much as 50% [1]. To date, attempts to engineer reduced oxygenase activity in RuBisCO have been largely unsuccessful.
Significantly, the cyanobacteria, eukaryotic microalgae, and C4 plants have evolved mechanisms to reduce photorespiration by concentrating CO2 near RuBisCO, competitively inhibiting oxygenase activity and leading to substantial increases in yield and water use efficiency per unit carbon fixed. However, carbon concentrating systems (CCMs) are not operational in the vast majority of plant species (i.e., C3 plants).
Attempts to reconstitute functional CCMs in C3 plants have been previously attempted by us and others, mainly focusing on engineering pathways that are directly involved in facilitating CO2 transport into leaf chloroplasts. Note, for example, PCT International Publication WO 2012/125737; Sage and Sage (2009) Plant and Cell Physiol. 50(4):756-772; Zhu et al. (2010) J Interg. Plant Biol. 52(8):762-770; Furbank et al. (2009) Funct. Plant Biol. 36(11):845-856; Weber and von Caemmerer (2010) Curr. Opin. Plant Biol.; Price (2013) J. Exp. Bot. 64(3):753-68; and U.S. Patent Application Publication No. 2013/0007916 A1.
However, ATP and NADPH production through light harvesting and electron transfer steps must be coordinated with carbon assimilation and additional energy requiring steps including CCM systems to prevent photoinhibition and to improve growth. Additionally, assimilatory flux and storage rates can limit carbon fixation due to feedback inhibition when sink demand is not matched to source capacity [2].
Thus, there is a critical need to improve plant productivity through integrated systems engineering approaches that balance source/sink interactions with energy and reductant production to develop energy-requiring, artificial CCMs that can effectively mimic those found in nature.